Gas turbine engines, such as those used for aircraft propulsion, typically include a bleed valve arrangement for diverting pressurized working medium fluid from a main flowpath. One common use of such a valve arrangement is to channel fluid, as necessary, from the engine's primary flowpath to a coannular secondary flowpath to temporarily improve the aerodynamic stability of the engine's compression system.
One general type of bleed valve arrangement is exemplified by U.S. Pat. Nos. 4,715,779 to Suciu (assigned to United Technologies Corporation, the assignee of the present application) and 3,638,428 to Shipley et al. In those arrangements the upstream edges of a series of circumferentially distributed doors are pivotably hinged to a frame, and a mechanical linkage extends from each door to a unison ring. Actuating means such as a mechanical actuator or a hydraulic motor are provided to displace the unison ring circumferentially thereby opening or closing the doors in parallel. However actuation forces can cause the unison rings or mechanical linkages to elastically deflect thereby compromising the capability to accurately position the doors. The unison rings are also susceptible to binding, especially if they become distorted as a result of nonuniform thermal expansion or aircraft maneuver loads. Moreover, net fluid pressure acting against each door produces a moment about the door hinge that must be overcome by the actuating system. Since the magnitude of the moment can be considerable, the actuating system components must be correspondingly bulky and heavy--characteristics that are clearly undesirable in aircraft engines.
Another type of widely used bleed valve arrangement is described in U.S. Pat. No. 4,827,713 issued to Peterson et al. and also assigned to United Technologies Corporation. In the disclosed arrangement, a valve ring is guided in a spiral motion between longitudinally spaced apart open and closed positions thereby controlling the flow of working medium gas through an array of circumferentially distributed passages. In the closed position, a pair of surfaces on the valve ring engages a corresponding pair of resilient seals to minimize leakage through the passages. Despite the many merits of this arrangement, deterioration and wear gradually degrade the effectiveness of the seals and diminish engine efficiency. Moreover, many of the components of the valve assembly are not readily accessible for repair or replacement without substantial engine disassembly.
The above described bleed valve arrangements may also not be readily adaptable to the newest generation of large diameter, high thrust turbine engines. These newer generation engines operate at higher pressures and temperatures than their older generation counterparts. The higher pressures would amplify the pressure induced moment about the door hinges of the arrangements attributable to Suciu and to Shipley et al., and the higher temperatures would lead to markedly accelerated deterioration of the resilient seals used in the bleed valve attributable to Peterson, et al.